Electrochemical sensors to determine whether the exhaust gases from internal combustion engines are reducing or oxidizing are well known and reference is made to U.S. Pat. No. 4,021,326, Pollner et al, which describes a sensor element, and its method of manufacture. The sensor element itself can be secured in a housing or socket for attachment to the exhaust system of an internal combustion engine, for example of an automotive-type engine, in such a way that the sensor element is exposed to the exhaust gases. U.S. Pat. Nos. 3,841,987, Friese et al, and 3,960,692, Weil et al, all assigned to the assignee of the present application, describe complete sensor constructions.
The sensor elements which are usually used, for example as described in the foregoing patents, are shaped in the form of a tube which is closed at one end, made of a solid electrolyte material. The thermodynamic gas balance is provided by an outer electrode which also acts as a catalyst. The electrode, preferably, is made of platinum or a platinum metal alloy. The platinum or platinum metal alloy electrode is in form of a porous layer which is usually applied by thermal vaporizing, vapor deposition, or cathodic atomization. The porous structure should be retained as long as possible in order to achieve a sufficiently high response speed during operation, which thereby increases the useful life of the sensor. To retain the porous structure, it has been proposed to apply a ceramic protective layer over the platinum or platinum metal alloy electrode. If the sensor, with only the porous electrode applied, is treated at an elevated temperature, for example at about 1000.degree. C. for an hour or so, then it will lose about 20% of its response speed. The reason for the drop-off of the response speed apparently is due to recrystallization processes which occur at this temperature, which leads to noticeable disappearance of a portion of the pores. The porous ceramic top cover layer as used heretofore has an inhibiting effect on the recrystallization processes, so that recrystallization is partly prevented. It has been observed, however, that the response speed will gradually drop as the temperature to which the sensor is exposed is increased.
It has previously been proposed to add dispersed metal oxides to metals in order to harden the metals; typically, such additives were made to increase the hardness of copper, gold or platinum, and thus hardened materials were then referred to as having been dispersion-hardened. The purpose of such dispersion hardening is to improve the strength of the metals at elevated temperatures, without substantial change in the other characteristics of the metals themselves. For example, materials which use dispersion-hardened platinum will retain their shape up to approximately the melting point of the platinum itself without otherwise losing physical or chemical properties thereof. The dispersion hardening can be obtained by alloying a few percent--by weight--of a metal oxide to the metal itself.